Atomic interactions in neon and helium

The first two quantum corrections to the second virial coefficients of the Smith-Thakkar potential are calculated. Parameters for neon and helium, gases in which quantum effects are important, are then determined by fitting to semiempirical dispersion coefficients and experimental second virial coefficients. Viscosity coefficients for both gases and vibrational energy level spacings for the neon dimer are calculated as independent tests of the potentials. Overall agreement with experiment is excellent for neon and moderate for helium. The previously determined parameters for argon are found to be only very slightly perturbed by the inclusion of quantum corrections in the calculated second virial coefficients.     

The direct determination of potential energy functions from second virial coefficients

Methods which enable the intermolecular pair potential energy curve to be determined from second virial coefficient data are critically examined. Those techniques which involve the inverse Laplace transform, though theoretically attractive, are found to be ineffective with experimental data. New approximate methods are proposed and found to be successful in characterizing both the repulsive and attractive regions of the potential energy function. The procedures were tested on simulated data calculated from a number of known potential energy functions and were shown to reproduce the original potentials with high accuracy. The techniques were then applied to experimental second virial coefficient data for argon to give the repulsive potential energy curve above 300 K and the width of the upper portion of the potential well as a function of its depth. When used in conjunction with similar techniques for the inversion of gas viscosity data, these methods enabled the potential energy curve for argon to be established over a wide range of intermolecular separation.     

Calculation of thermodynamic properties of vapor–liquid equilibria using ab initio intermolecular potential energy surfaces for dimer O2–O2

The two 5-site potentials from ab initio calculations at the theoretical level CCSD(T) with correlation consistent basis sets aug-cc-pVmZ (with m?=?4, 34) have been constructed from oxygen. The extrapolation ab initio energies were approximated by the basis sets aug-cc-pVmZ (m?=?3, 4). These two potentials were constructed by using the ab initio intermolecular energy values and a non-linear least-squares fitting method. The second virial coefficients of oxygen were determined to demonstrate the accuracy of these ab initio 5-site potentials. These ab initio potentials were employed to estimate the thermodynamic properties of the vapor–liquid equilibria by GEMC simulation. The influence of ab initio potential alone and plus 3-body interaction Axilrod-Teller potential was investigated within GEMC simulation from 80?K to 140?K. The discrepancy between them is insignificant. This showed that the two 2-body 5-site potential functions can also be used together with the 3-body interaction Axilrod-Teller potential to generate the accurate thermodynamic properties of the liquid–vapor equilibria.     

Editorial

Using coupled cluster singles and doubles linear response theory and the d-aug-cc-pVTZ basis set extended with a 3s3p2d1f1g set of midbond functions, the interaction induced electric dipole polarisability surface of the CO–Ar van der Waals complex is computed. Combining this surface with accurate intermolecular potential energy and electric dipole surfaces, the pressure and dielectric second virial coefficients of the complex are calculated by a classical statistical approach. Excellent agreement with experimental results (to within the experimental error bars) is obtained for the pressure second virial coefficient over a range of temperatures. No previous experimental or theoretical investigations have been carried out for the dielectric second virial coefficient, B _ε(T), which is estimated to be about 1.9 cm⁶ mol^??1 at room temperature. This value results from a balance of terms due to the interaction induced electric dipole polarisability (predominant at high temperatures) and orientational electric dipole contributions.     

The intermolecular potential energy surface of Ar · HC1

Anisotropic potential energy surfaces for Ar · HC1 are obtained by simultaneous least squares fitting to molecular beam spectra, rotational line broadening cross sections, second virial coefficients and molecular beam total differential cross sections. Several potentials are obtained which given good agreement with all these data and the sensitivity of the data to various aspects of the potential is investigated. For all potentials the equilibrium configuration is linear with the atomic arrangement Ar · H-C1. Several different ways of parameterizing the intermolecular potential are considered and it is concluded that the conventional multipole parameterization is not adequate for strongly anisotropic intermolecular potentials.     

On the validity of the triple-dipole interaction as a representation of non-additive intermolecular forces

A complete partial wave analysis of the non-expanded non-additive coulomb interaction energy for three non-degenerate S-state atoms is given through third-order in the interatomic potential energy function. Pseudo state techniques are used to evaluate various partial wave components of the non-expanded second and third-order non-additive interaction energies for various isosceles triangular configurations of three interacting ground-state hydrogen atoms. These second and third-order non-expanded coulomb results are used, in conjunction with Heitler-London results for the first-order non-additive energies for the quartet spin state of the H(1s)-H(1s)-H(1s) interaction, to discuss the relative importance of various parts of the non-additive energy as a function of the geometrical configuration of the atoms, and the validity of both the non-expanded triple-dipole energy and the expanded Axilrod-Teller-Muto triple-dipole result as a representation of non-additive coulomb energies. For example, in the non-bonded interaction of three S-state atoms it appears that representing the non-additive energy by the non-additive coulomb energy is not reliable until the interatomic separations are somewhat larger than R*, the interatomic distance associated with the van der Waals minimum in the corresponding non-bonded dimer interaction. Further, the use of the triple-dipole interaction energy, with or without charge overlap corrections, to represent the non-additive coulomb energy is of doubtful validity until the interatomic separations are considerably greater than R*.     

Pressure Lineshift and Broadening Coefficients in the 2ν3 Band of OCS

In this study we report the first measurements of the pressure-induced lineshift coefficients due to Ar, He, O₂, and N₂ for 22 rovibrational lines from P(53) to R(53), belonging to the 2ν₃ band of ¹⁶O¹²C³²S at 4100 cm⁻¹. The lineshift results were obtained from the simultaneous record of the pressure-broadened and pure low-pressure OCS lines, using a tunable difference-frequency laser spectrometer. For four lines of the 2ν₃ band we also report Ar-, He-, O₂-, and N₂-broadening coefficients by fitting Voigt and Rautian profiles to the measured shapes of these lines. The broadening and shift coefficients are compared to the results of theoretical calculations based on the semiclassical Robert–Bonamy formalism and two different isotropic and anisotropic intermolecular potentials. For OCS–Ar we also consider the Smith–Giraud–Cooper model including all orders of the interaction within the peaking approximation. In all cases, the calculated broadening coefficients are in reasonable agreement with the experimental data. By considering adjustable parameters for the vibrational dependence of the isotropic potential, the general trends of the lineshifts with J can be roughly predicted, except at low J values where an asymmetry behavior for P and R branches is generally observed.     

The influence of intermolecular interactions on the Kerr effect in gases

Expressions are derived for the second and third Kerr virial coefficients, B _K and C _K, of spherical top molecules in terms of irreducible cluster polarizabilities, and values are calculated using the dipole-induced dipole model for argon, krypton, xenon, methane, tetrafluoromethane, neopentane and sulphur hexafluoride. For mixtures of rare gases it is shown that the collision-induced dipole moment makes a negligible contribution to B _K. The effect of the choice of intermolecular potential function on the calculated second Kerr virial coefficients is also demonstrated. It is found that the predominant contributions to C _K arise from the pair polarizability, and that the triplet polarizability is only of minor importance.     

Ab initio intermolecular potential energy surface of Ne···NCCN van der Waals complex: effect of the place of midbond function on the interaction

The intermolecular potential energy surface of Ne···NCCN van der Waals complex was evaluated in the framework of the counterpoise-corrected supermolecular approach using CCSD(T) level and aug-cc-pVDZ basis set extended with a set of midbond (3s3p2d1f1g) functions. The effect of the place of midbond function on the accuracy of the calculated potential energy surface was examined and the optimised position for placing midbond function was determined. The calculated potential energy surface was fitted by an analytical function. The analytical function of intermolecular potential energy surface of Ne···NCCN demonstrated a global minimum energy of ?12.024 meV related to the T-shape geometry at the distance between Ne and the centre of mass of NCCN of 3.28 Å. Finally, the interaction second virial coefficients (B₁₂) of Ne and NCCN were calculated and used to calculate the second virial coefficients for the mixture of neon and cyanogen gases at different mole fractions of Ne gas.     

Exchange-Coulomb potential energy surfaces and related physical properties for Ne-N2

An exchange-Coulomb (XC) potential energy model is developed for the Ne-N₂ interaction. The construction of this new potential energy surface is based on recent results for the Heitler-London interaction energy, the long range dispersion energies, and the microwave spectra of the dimer. The adjustable parameters in the final XC1 potential energy surface have been determined by fitting the frequencies of three representative lines of the microwave spectrum for the two isotopomers ²⁰Ne-¹⁴N₂ and ²⁰Ne-¹⁵N₂ while simultaneously maintaining agreement with the experimental second virial coefficient data obtained with the initial (unadjusted XC0 potential. With no further adjustment of parameters, the final XC potential reproduces, within 0.005%, the frequencies of the 34 microwave transitions studied experimentally for four isotopomers of Ne-N₂. Excellent agreement with experiment is obtained also for the binary diffusion, the interaction viscosity, and the mixture viscosity (for all compositions) coefficients for all temperatures. Agreement with experiment for the relaxation cross-sections associated with viscomagnetic effects, and with the pressure broadening of depolarized Rayleigh light scattering, is very good given the level of computation used for the calculations and the accuracy with which some of these cross-sections currently can be determined from experimental data. Comparisons are made with predictions calculated from the three best literature potential energy surfaces or Ne-N₂. The final XC1 potential energy surface is overall the most reliable potential energy for this van der Waals complex to date. The flexibility still inherent in this potential can be exploited, if required, in future studies of the Ne-N₂ system.     

The intermolecular potential energy functions of krypton and xenon

The intermolecular potential energy functions for krypton and xenon have been determined using new semi-inversion techniques. These techniques, which have previously been applied to the data for argon, enable information about intermolecular forces to be obtained directly from second virial coefficient and gas viscosities measurements.     

A critical evaluation of isotropic potential functions for chlorine

A survey of many crystal structures shows that short intermolecular chlorine-chlorine contacts are anisotropic. This observation challenges the currently used potential function models which allow only for isotropic exp-6 and coulomb interactions. In this study the polymorphic p-dichlorobenzene DCB system is used to illustrate the difficulties encountered by relying on the isotropic atom-atom approximation. It is found that exp-6 potential functions cannot be extrapolated beyond the range of contacts used in their derivation. Addition of coulomb terms provides needed additional flexibility to the potential function but the derived C-Cl bond polarization is not found to be a constant of the molecule, i.e. the optimum value is not the same for each phase of DCB. Furthermore, no single exp-6 potential function could satisfy the structural constraints provided by the three phases of crystalline DCB and by hexachlorobenzene. These findings are typical of curve-fitting methods which employ an incorrect mathematical form for the curve and indicate the necessity of including anisotropic terms in the interaction potential.     

Ab initio intermolecular potential energy surfaces for the Ar–NCCN van der Waals complexes

The intermolecular potential energy surface of complex pairing argon with cyanogen molecule (NCCN) was calculated using the coupled cluster with single and double and perturbative triple excitations (CCSD(T)) with aug-cc-pvdz basis set extended with a set of mid-bond (3s3p2d1f1g) functions. The interaction energies were calculated by the supermolecular approach with the full counterpoise correction for the basis set superposition error. The calculated potential energies were fitted to an analytical expression. The calculated Ar–NCCN potential energy surface shows a global minimum at 3.35 Å, the distance between argon and centre of mass of cyanogen, for the T-shaped geometry and two local minimum at distance of 5.54 Å for the linear geometry on one side of cyanogen. Finally, the interaction second virial coefficients were calculated using the fitted potential energy surface and were compared with those obtained by the parameters of the Beattie–Bridgeman equation of states of pure argon and cyanogens fluids, approximately.     

Atom-atom potential in rotational line broadening for molecular gases: Application to CO lines broadened by CO,N2, O2 and NO

Linewidths in CO-CO, CO-N₂, CO-C₂ and CO-NO collisions have been calculated using an improved potential for atom-atom interactions in addition to the first-order terms of the electrostatic interaction. The energy parameters characterizing the Lennard-Jones potential have been estimated previously from second virial coefficients. Overall agreement between calculated and experimental linewidths (particularly for CO-O₂ mixtures for which the electrostatic contributions are weak) is thus obtained without any adjustable parameters. It is concluded that the atom-atom potential describes the angular dependence of the short-range forces in molecular collisions fairly adequately.     

Measurements and calculations of ethylene line-broadening by argon in the ν7 band at room temperature

Argon broadening coefficients are measured for 32 vibrotational lines in the ν₇ band of ethylene at room temperature using a tunable diode-laser spectrometer. These lines with 3 ≤ J ≤ 19, 0 ≤ K_a ≤ 4, 2 ≤ K_c ≤ 19 in the P, Q and R branches are located in the spectral range 919–1023 cm^?1. The fitting of experimental line shapes with Rautian profile provides collisional widths slightly larger than those derived from the Voigt profile. The independent theoretical estimation of these line widths is performed by the semiclassical approach of Robert-and-Bonamy type with exact isotropic trajectories generalized to asymmetric tops. Even with a rough atom–atom intermolecular potential model the calculated values show good agreement with experimental results.     

Ab initio pair potential energy curve for the argon atom pair and thermophysical properties for the dilute argon gas. II. Thermophysical properties for low-density argon

A recent argon–argon interatomic potential energy curve determined from quantum-mechanical ab initio calculations and described with an analytical representation [B. Jäger, R. Hellmann, E. Bich, and E. Vogel, Mol. Phys. 107, 2181 (2009); 108, 105 (2010)] was used to calculate the most important thermophysical properties of argon governed by two-body interactions. Second pressure, acoustic, and dielectric virial coefficients as well as viscosity and thermal conductivity in the limit of zero density were computed for natural argon from 83 to 10,000?K. The calculated values for the different thermophysical properties are compared with available experimental data and values computed for other argon–argon potentials. This extensive analysis shows that the proposed potential is superior to all previous ones and that the calculated thermophysical property values are accurate enough to be applied as standard values for the complete temperature range of the calculations.     

The intermolecular potential of NH+ 4-Ar I. Calculations for the internal rotor structure of the v 3 band

The intermolecular potential energy surface of the electronic ground state of the ammonium-argon ionic dimer, NH⁺ ₄-Ar, is calculated by ab initio methods using different levels of theory (MP2, MP4) and basis sets (aug-cc-pVXZ, X = D/T/Q). The deformation of the ammonium ion in the complex is shown to be small and its geometry is therefore fixed in these calculations to the tetrahedral structure optimized for the bare ion. The global minimum of the potential corresponds to a proton-bound structure with C_3v symmetry (R_e ≈ 3.4 Å, D_e ≈ 950 cm^?1) and the barrier to internal rotation between the four equivalent minima is around 200 cm^?1. The three-dimensional potential is expanded in tetrahedral harmonics whose radially dependent coefficients, V_i (R), are compared for the considered levels of theory. The rotation-intermolecular vibration Hamiltonian is solved using a two-dimensional fixed-R representation of the calculated potentials, V_i , ≡ V_i (R ^eff), where the effective intermolecular separation, RR^eff, is determined from the experimental rotational constants of the complex. The accuracy of these parametrized potential energy surfaces is judged by their ability to reproduce the hindered rotor subband structure in the experimental v ₃(t ₂) infrared band of the complex. The simulations using the potentials calculated at the MP2/aug-cc-pVTZ or higher levels of theory reproduce the coarse structure of the experimental spectrum well. Further improvement could be achieved by least-squares fitting the potential parameters to the observed subband positions. The fitted V ₃ and V ₄ parameters remain in close agreement with those determined from the ab initio calculations but the anisotropy of the potential is significantly different from that in a previous least-squares fit of V ₃ alone.     

Accurate second Kerr virial coefficient of rare gases from the state-of-the-art ab initio potentials and (hyper)polarizabilities

The second Kerr virial coefficient of rare gases is studied in this work using the best ab initio potentials and (hyper)polarizabilities in the literature. The second Kerr virial coefficient of helium-4, helium-3, neon, argon, and krypton and its polarizability component of xenon are computed by the semi-classical method together with the Padé approximant over a wide temperature range. In addition, the uncertainty of second Kerr virial coefficient is estimated from the uncertainties of the ab initio interaction-induced properties. The experimental and theoretical data in the literature is compared with our calculated values to examine the quality of this work. It is shown that our computed values in the supplementary materials are as accurate as the literature data at medium and high temperatures and are more reliable at low temperatures.     

稀有气体纯质热物理性质的预测

根据量子力学和分子运动学理论,采用稀有气体的ab initio势能,分别计算了氦-4、氖、氩、氪和氙纯质在低密度时的热物理性质,包括第二维里系数,热扩散系数和热扩散因子,计算的温度范围为50—5000 K.预测结果具有较高的精度,与采用经验势能的计算结果相比,本文结果更接近实验数据和REFPROP 8.0的标准值,为相关的科学研究和工程应用提供了所需的基础数据. 关键词： ab initio势能')" href="#">ab initio势能 稀有气体 热物理性质     

An approximate multi-property empirical isotropic interatomic pair potential for the krypton dimer

An approximate empirical isotropic interatomic potential for krypton interaction is developed by simultaneously fitting the Morse-Morse-Morse-Spline-van der Waals potential form to the pressure second virial coefficient, viscosity, thermal conductivity and depolarized interaction-induced light scattering data. Absolute zeroth and second moments of the two-and three-body spectra, the pressure third virial coefficient and isotopic thermal diffusion factor have been measured and compared with theoretical calculations using various models for the interatomic potential. The results show that it is the most accurate potential yet reported for this system. The use of the new potential in lattice sum calculations yields good results for several properties of solid krypton.